The goal of this project is to study antigenic variation of equine infectious anemia virus (EIAV) with regard to possible mechanisms of viral persistence through avoidance of the specific immune response. A focal immunofluorescence assay has been used to expand and biologically clone a number of viral isolates in equine, feline and canine cell lines. In addition, viral isolates from sequential febrile periods have been isolated from infected horse blood, have been adapted to replicate in an equine dermal cell line and have been biologically cloned in that cell line. One of these field isolates, MA-1, is being used to infect a horse in order to generate antigenic variants in vivo. Monoclonal antibodies are being developed in order to antigenically characterize the various viral isolates. A total of 34 clones has been found reactive with EIAV-infected cells by membrane and/or cytoplasmic fluorescence. Further characterization of these clones is ongoing. Virus isolates are also being compared by restriction enzyme analyses of Hirt supernatant fractions of infected cells.